1. Field of the Invention
The invention relates generally to the field of wellbore drilling methods and apparatus. More specifically, the invention relates to methods for drilling wellbores along a selected trajectory which are directionally stable.
2. Description of the Related Art
Wellbore drilling of earth formations for such purposes as petroleum extraction includes rotating a drill bit while applying axial force against the bit. Systems for applying the axial force and rotating the bit include drilling rigs which may directly rotate a string of drill pipe coupled to the drill bit. Alternatively or additionally, various types of hydraulically or pneumatically operated motors can be coupled to the bit. These so-called “mud motors” are operated by pumping drilling fluid through them. Generally, there are two basic types of mud motors. One type of motor is called “positive displacement”. Positive displacement motors include a chambered stator in the interior of the motor housing which is usually lined with an elastomeric material, and a rotor which is rotationally coupled to the motor output shaft (and thence to the drill bit). Movement of drilling fluid through chambers defined between the stator and rotor causes the rotor to turn correspondingly to the volume of fluid pumped through the motor. The other type of mud motor is called “turbine”, because the output of the motor is coupled to a turbine disposed inside the motor housing.
It is known in the art to drill a wellbore along a selected trajectory for the purpose of penetrating certain subsurface earth formations which are displaced from the surface location of the wellbore. Collectively, the techniques used to drill along such trajectories are known as directional drilling. Directional drilling includes using mud motors includes using motors having housings which are bent to a predetermined angle. These motors cause the trajectory of the wellbore to turn in the direction of the interior of the bend in the housing. One class of such motors is called “steerable”, wherein the predetermined angle is about ½ degree. When the housing of a steerable motor is rotated by the drilling rig, drilling the wellbore progresses so as to substantially maintain the existing trajectory of the wellbore. This is called “rotating” or “rotary drilling”.
To adjust the trajectory of the wellbore using a steerable motor, the rotation of the motor housing by the rig is stopped, and drilling continues only by the rotation of the motor output shaft. This is referred to as “slide drilling”. As with other types of bent housing motors, slide drilling with a steerable motor causes the wellbore trajectory to turn in the direction of the bend in the motor housing.
It is very important in all types of directional drilling for the drilling assembly, including the drill bit, to be dynamically stable so that the selected trajectory will be maintained.
A particular type of drill bit, generally called a “bi-center” bit, has proven useful in certain drilling environments. A bi-center bit generally is characterized by having a “pass-through” diameter (the smallest diameter opening through which the bit will freely pass) which is smaller than the diameter of the hole drilled by such bits when they are rotated. Bi-center bits have proven useful, among other applications, when drilling through earth formations which are subject to swelling, or for drilling where it is desirable to be able to insert a pipe or casing in the drilled wellbore which has an outside diameter as close as possible to the inside diameter of a previously set casing.
Several types of bi-center drill bits are known in the art. One such bi-center bit is described in U.S. Pat. No. 5,678,644 issued to Fielder, for example. The bit shown in the Fielder '644 patent includes a pilot section, and a separate reaming section which drills a hole having the full drill diameter. Another type of bi-center bit is sold under the trade name “Steering Wheel Bi-Centrix”, by Reed-Hycalog division of Schlumberger, Houston, Tex. Another type of bi-center bit is sold under the trade name Quad-D by Smith International, Inc., the assignee of this invention. These bits also have a pilot section and a reaming section. Another type of bi-center bit is known as “asymmetric” and does not include a separate reaming section, but instead has one side which conforms to a drill radius. The other side of the asymmetric bit conforms to a smaller radius to provide the bit with a pass-through diameter which is smaller than the drill diameter. One such asymmetric bit is sold under the trade name “X-TEND” by Smith International, Inc., the assignee of the present invention.
Bi-center bits have been difficult to use when directional drilling, because they can introduce directional instability in the drilling assembly when directional drilling. One reason for the instability is that it is not possible to use stabilizers in the drilling tool assembly near a bi-center drill bit. The instability is particularly noticed when drilling with mud motors, and more particularly, positive displacement mud motors. It is desirable to provide a drilling technique that is directionally stable when using mud motors with bi-center drill bits.